An electromagnetic flowmeter measures a flow rate of electrically conductive fluid to be measured, which flows through a measuring pipe, by converting the flow rate into an electrical signal using an electromagnetic induction phenomenon. FIG. 27 shows a conventional general-form electromagnetic flowmeter. The electromagnetic flowmeter includes: a measuring pipe 1 through which fluid to be measured flows; a pair of electrodes 2a and 2b that are provided in the measuring pipe 1 so as to face each other, so as to be perpendicular to both a magnetic field which is applied to the fluid to be measured and an axis PAX of the measuring pipe 1, and so as to be in contact with the to-be-measured fluid, and detects an electromotive force generated by the magnetic field and the flow of the fluid to be measured; an exciting coil 3 that applies, to the fluid to be measured, the magnetic field that is perpendicular to both an electrode axis EAX connecting the electrodes 2a and 2b and the measuring pipe axis PAX; a signal conversion unit 5 that detects an electromotive force between the electrodes 2a and 2b; and a flow rate output unit 6 that calculates the flow rate of the fluid to be measured on the basis of the inter-electrode electromotive force detected by the signal conversion unit 5.
In the general-form electromagnetic flowmeter shown in FIG. 27, when a plane PLN that is perpendicular to the direction of the measuring pipe axis PAX and includes the electrodes 2a and 2b, is regarded as a boundary in the measuring pipe 1, a magnetic field that is symmetrical with respect to the plane PLN, which is the boundary in the measuring pipe 1, is applied to the fluid to be measured. Further, examples of the excitation system of the exciting coil 3 include a sine wave excitation system that enables high frequency excitation, and a rectangular wave excitation system that is not affected by electromagnetic induction noise and the like.
The sine wave excitation system that uses a sine wave as exciting current of the exciting coil 3 is likely to be affected by commercial frequency noise, but this drawback can be solved by the high frequency excitation system in which the frequency of exciting current is made high. Further, there are advantages that the high frequency excitation system is resistant to 1/f noise such as electrochemical noise or spike noise, and responsiveness (a characteristic of causing a flow rate signal to quickly follow a change in the flow rate) can be also improved.
However, the conventional sine wave excitation system is likely to be affected by noise of an in-phase component. An example of the noise of the in-phase component is a shift of the amplitude of a magnetic field applied to the fluid to be measured. In the conventional electromagnetic flowmeter, when the amplitude of exciting current supplied to the exciting coil 3 changes (shifts) due to variation of a power supply voltage or the like so that the amplitude of the magnetic field applied to the fluid to be measured shifts, the amplitude of the electromotive force between the electrodes changes, and an error occurs in flow rate measurement due to the effect of the shift. Such noise of the in-phase component cannot be eliminated even by using the high frequency excitation system.
On the other hand, the rectangular wave excitation system that uses a rectangular wave as exciting current supplied to the exciting coil 3 is resistant to the noise of the in-phase component. However, in the rectangular wave excitation system, an electromotive force between electrodes is detected at time when a change in the magnetic field stops. However, in case of the exciting current with a high frequency, a detector is required to have high performance. In addition, in the rectangular wave excitation system, in case of the exciting current with high frequency, effects of, the impedance of the exciting coil 3, the responsiveness of the exciting current, the responsiveness of the magnetic field, overcurrent loss at the core of the exciting coil 3 and the measuring pipe 1, and the like, cannot be ignored, and it is difficult to maintain the rectangular wave excitation. As a result, in the rectangular wave excitation system, it is difficult to achieve the high frequency excitation, and improvement of responsiveness with respect to a change in the flow rate and elimination of 1/f noise cannot be achieved.
In addition, since a flow rate is a product of a flow speed and a cross-sectional area of the measuring pipe, the flow rate and the flow speed generally have one-on-one relationship at calibration in an initial state, and obtaining the flow speed is regarded as being equivalent to obtaining the flow rate. Thus, a method for obtaining a flow speed (in order to obtain a flow rate) will be described below.
As an electromagnetic flowmeter that can correct an error in flow rate measurement by eliminating the noise of the in-phase component and can achieve high frequency excitation, the inventors have proposed an asymmetrical excitation electromagnetic flowmeter as shown in FIG. 28 (see Japanese Patent No. 3774218 and Japanese Patent Application Publication No. 2005-300325). Unlike the general-form electromagnetic flowmeter shown in FIG. 27, the asymmetrical excitation electromagnetic flowmeter shown in FIG. 28 extracts a parameter (asymmetrical excitation parameter) that is not affected by a shift of a span, and outputs a flow rate on the basis of the parameter, thereby solving the problem concerning the shift of the span.
Here, a shift of a span will be described with reference to FIG. 29. If the magnitude V of a flow speed measured by an electromagnetic flowmeter changes even when the flow speed of fluid to be measured does not change, it is thought that a shift of the span is a cause of the output change. For example, it is assumed that, in an initial state, the electromagnetic flowmeter is calibrated such that: the output of the electromagnetic flowmeter is 0 (v) when the flow speed of the fluid to be measured is 0; and the output is 1 (v) when the flow speed is 1 (m/sec). The output of the electromagnetic flowmeter is a voltage that represents the magnitude V of the flow speed. Because of such calibration, when the flow speed of the fluid to be measured is 1 (m/sec), the output of the electromagnetic flowmeter should be 1 (v). However, at the time when a certain time t1 has elapsed, the output of the electromagnetic flowmeter may be 1.2 (v) even though the flow speed of the fluid to be measured is maintained at 1 (m/sec). The reason for this output change is thought to be a shift of the span. The phenomenon of the shift of the span occurs, for example, because the value of the exciting current flowing in the exciting coil cannot be maintained at a constant value due to a change in the ambient temperature of the electromagnetic flowmeter.
However, unlike the general-form electromagnetic flowmeter, an offset needs to be provided between the electrode position and the coil position in an asymmetrical excitation electromagnetic flowmeter as in the configuration shown in FIG. 28. Thus, a detector for use in the general-form electromagnetic flowmeter cannot be used in the asymmetrical excitation electromagnetic flowmeter, and it is necessary to newly design and produce a detector part.